fix(developers): distinguish API errors from taken usernames in availability check

CheckUsernameAsync returned false (taken) on any API failure, making every
username appear taken when SilverDESK was unreachable. Now returns nullable
bool so errors show a warning instead of blocking submission.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

BlazorApp/wwwroot/developers-styles.css

@@ -186,6 +186,29 @@
     margin-top: 0.3rem;
 }
 
+/* Username status */
+.username-status {
+    display: block;
+    font-size: 0.82rem;
+    margin-top: 0.35rem;
+}
+
+.username-checking {
+    color: rgba(255, 255, 255, 0.5);
+}
+
+.username-available {
+    color: #34d399;
+}
+
+.username-taken {
+    color: #f87171;
+}
+
+.username-error {
+    color: #fbbf24;
+}
+
 /* Validation messages */
 .validation-message {
     color: #f87171;